If you're using stackmail, I've read it should be as follows unless you've been instructed to otherwise by the provider:
SMTP server: smtp.stackmail.com
Encryption: STARTTLS (do NOT use the if available option)
Username/password: your email address and associated password
Also, at the bottom of the screen, the setting for "Login Credentials for Email Submission" should be set to predefined, not logged-in user too.
What the previous poster said plus probably try port 587 and StartTLS. SSL and port 465 is pretty much all but invalid these days. A lot of companies that used those settings have since switched to Port 587 and StartTLS.
I cannot resolve the servername in your screenshot. I can resolve smtp.crestico.net though. Try changing your hostname to that address.
I also recommend changing your port to 587 and the encryption type to STARTTLS. Port 465 is still open on that server, but most providers have moved on from using port 465 with the older implementation of SSL. It's for the best to start using a more up to date standard while you're making changes.
Scan to eMail function
My device started giving this error message two days. there has been no changes in my home office network and I can't figure out what could be the issue.